「BZOJ3160」万径人踪灭

注意manacher的写法 注释掉的是TLE的代码

 1 #include<bits/stdc++.h>
#define ll long long
#define db double
using namespace std;
const int N=150010;
const ll mod=1e9+7;
const db PI=acos(-1);
struct Complex{
    db real,image;
    Complex(db _real=0,db _image=0):real(_real),image(_image){}
    Complex operator+(Complex k)const{return Complex(real+k.real,image+k.image);}
    Complex operator-(Complex k)const{return Complex(real-k.real,image-k.image);}
    Complex operator*(Complex k)const{return Complex(real*k.real-image*k.image,real*k.image+image*k.real);}
    Complex operator/(int k)const{return Complex(real/k,image/k);}
}a[N<<1],b[N<<1];
int n,m,l,maxp,f[N<<1],rev[N<<1],pow2[N<<1];
char s[N];
void fft(Complex* x,int f){//f=1时为fft，f=-1时为ifft
    for(int i=0;i<n;i++) if(rev[i]>i) swap(x[i],x[rev[i]]);
    for(int len=1;len<n;len<<=1){
        Complex wn(cos(PI/len),f*sin(PI/len));
        for(int now=0;now<n-1;now+=(len<<1)){
            Complex w(1,0);
            for(int i=0;i<len;i++,w=w*wn){
                Complex t1=x[now+i],t2=w*x[now+len+i];
                x[now+i]=t1+t2,x[now+len+i]=t1-t2;
            }
        }
    }
    if(f==-1) for(int i=0;i<n;i++) x[i]=x[i]/n;
    return;
}
int maxr,pos,r[N<<1];
//int manacher(){
//  int ans=0;
//  s2[l2++]='#';
//  for(int i=0;i<l;i++) s2[l2++]=s[i],s2[l2++]='#';
//  for(int i=0;i<l2;i++){
//      r[i]=(maxr>=i)?min(maxr-i+1,r[(pos<<1)-i]):1;
//      while(i-r[i]>=0&&i+r[i]<l2&&s2[i-r[i]]==s2[i+r[i]]) r[i]++; 
//      if(r[i]>maxr) maxr=r[i],pos=i;
//      ans=ans+(r[i]>>1);
//      if(ans>mod) ans-=mod;
//  }
//  return ans;
//}
int manacher(){
    int ans=0;
    s[l+1]='#',s[0]='$';
    for(int i=1;i<=l;i++){
        if(maxr>i) r[i]=min(maxr-i+1,r[pos*2-i]);
        else r[i]=1;
        while(s[i+r[i]]==s[i-r[i]]) r[i]++;
        if(i+r[i]-1>maxr) maxr=i+r[i]-1,pos=i;
        ans=(ans+r[i])%mod;
    }
    maxr=0;
    for(int i=1;i<=l;i++){
        if(maxr>i) r[i]=min(maxr-i,r[pos*2-i]);
        else r[i]=0;
        while(s[i+r[i]+1]==s[i-r[i]]) r[i]++;
        if(i+r[i]>maxr) maxr=i+r[i],pos=i;
        ans=(ans+r[i])%mod;
    }
    return ans;
}
int main(){
    scanf("%s",s+1);
    l=strlen(s+1),n=l-1;
    m=n<<1;
    for(n=1;n<=m;n<<=1) maxp++;
    for(int i=1;i<n;i++) rev[i]=(rev[i>>1]>>1)|((i&1)<<(maxp-1));
    for(int i=1;i<=l;i++) a[i-1].real=(s[i]=='a'),b[i-1].real=(s[i]=='b');
    fft(a,1);fft(b,1);
    for(int i=0;i<n;i++) a[i]=a[i]*a[i],b[i]=b[i]*b[i];
    fft(a,-1);fft(b,-1);
    pow2[0]=1,f[0]=int(a[0].real+b[0].real+1.1)>>1;
    for(int i=1;i<n;i++){
        pow2[i]=pow2[i-1]<<1,f[i]=int(a[i].real+b[i].real+1.1)>>1;
        if(pow2[i]>=mod) pow2[i]-=mod;
    }
    ll ans=0;
    for(int i=0;i<n;i++) ans=(ans+pow2[f[i]]-1)%mod;
    int temp=manacher();
    ans-=temp;
    if(ans<0) ans+=mod;
    printf("%lld",ans);
    return 0;
}